toolchain helpers: introduce simplify_symlink

The external toolchain logic flattens the directory layout in the staging
directory.  Regardless of the number of levels present in the extracted
toolchain, libraries are always copied to lib/ and usr/lib/, and directory
symbolic links are provided to make the original paths available as well.

Due to this, the same library may be reachable through a number of paths:
one path without any symbolic link, and one or more paths using directory
symlinks.

Using a direct path in a symlink destination is generally preferred because
it is clearer, but it is also more robust against accidental removal of an
intermediate directory symlink.

Introduce a helper function to simplify a symlink's destination to such a
direct path. This function will be used in a subsequent patch.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>